1. Field of the Invention
This invention relates generally to the field of gas turbine engines for the generation of electricity and, more particularly, to a method for controlling exhaust gas temperature of a gas turbine engine after reaching self-sustaining speed but prior to reaching synchronous speed.
2. Description of the Prior Art
Gas turbine engines are utilized to drive a rotor or drive shaft in a turboalternator. The rotation of the rotor or drive shaft coupled with electronic devices creates electric power as is known in the art. The starting of a gas turbine engine is a complex operation. Typically, before the gas turbine engine is run on its own power, the engine must be accelerated by an external electric source, such as a battery, to provide sufficient airflow to the combustor for ignition. In a turboalternator having a permanent magnet rotor/generator coupled to a gas turbine engine, supplying electrical power to the permanent magnet rotor/generator will have it function as a motor to drive the gas turbine engine. Typically, engine speed varies as a function of the torque versus speed characteristics of the starter motor.
Prior to ignition, a fuel/air mixture is created in the combustion chamber. Once the correct fuel-to-air ratio is achieved, the fuel/air mixture in the combustion chamber is ignited. At this point, the gas turbine engine is driven by a combination of the power exerted from the external electric source and the power created by combustion within the combustion chamber of the gas turbine engine. The power supplied by the combination of the external electric source and the combustor within the combustion chamber continues to accelerate the gas turbine engine until it reaches self-sustaining speed. After self-sustaining speed has been reached, the external electric source is usually disabled, and the additional acceleration required for the gas turbine engine to reach synchronous speed is supplied internally by combustion.
Once the external electric source has been disabled, it is advantageous to control the acceleration of the gas turbine engine until synchronous speed is reached. While the gas turbine engine continues to accelerate, it is also advantageous to make sure that the exhaust gas temperature (EGT) does not reach a temperature level that could damage the turboalternator.
A prior art method of controlling the gas turbine engine after the external electric source has been disabled uses an open-loop control system. Before starting the gas turbine engine, two EGT levels are defined: (1) a maximum start EGT; and (2) a maximum operational EGT. The xe2x80x9cmaximum start EGTxe2x80x9d refers to a level of EGT to limit below during the start up sequence of the gas turbine engine. The xe2x80x9cmaximum operational EGTxe2x80x9d refers to the maximum EGT to limit below after achieving synchronous speed. By xe2x80x9cmaximum EGTxe2x80x9d is meant the maximum level of EGT that can be allowed for operation before shutting down the gas turbine engine. If the actual EGT is above the maximum start EGT when entering the stage of operation between self-sustaining speed and synchronous speed, the gas turbine engine will shut down. If the actual EGT is not above the defined maximum start EGT when entering the stage of operation between self-sustaining speed and synchronous speed, the fuel valve is opened until the actual EGT reaches the maximum operational EGT. Once the maximum operational EGT is reached, the fuel valve is controlled around the maximum operational EGT using an open-loop controller.
The above prior art method for controlling the gas turbine engine after the electric source has been disabled commonly results in either: (1) improper control of acceleration; or (2) overheating in the combustion chamber causing the gas turbine engine to shut down and, possibly, causing damage to the gas turbine engine. These results occur because of the inherent instability involved in open-loop control.
It is, therefore, an aspect of the present invention to avoid overheating of the gas turbine engine by providing a method for controlling fuel flow to the combustor, thereby limiting the EGT.
The method for controlling the acceleration rate and EGT of a gas turbine engine of the present invention utilizes a turbine compressor, an annular combustor, and a control system containing two PID controllers. The annular combustor can include a single fuel source or multiple fuel sources.
Prior to operating a gas turbine engine, the following steps should be taken: (1) defining a moderate EGT; and (2) creating a table of acceleration rates. The table should be a three-dimensional table which utilizes turbine engine speed, inlet temperature, and EGT as its parameters. By xe2x80x9cmoderate EGTxe2x80x9d is meant a certain level of EGT that is as low as operationally possible that will limit exposure of the turbine engine to high temperature.
In order to maintain the operation of the turbine gas engine, compressed air and fuel are continually fed to the combustion chamber. Next, the external electric source is disabled upon a sensing device sensing that the gas turbine engine has reached self-sustaining speed. After the external electric source is disabled, the method of operating a gas turbine engine involves the following steps: (1) enabling a timing device; (2) monitoring the EGT, speed of the engine rotor or turbine drive shaft, the acceleration rate of the engine rotor or turbine drive shaft, and the inlet temperature; (3) requesting an acceleration rate from the created table; (4) enabling a PID controller to request a fuel valve position based upon the requested acceleration rate and the actual acceleration rate of the gas turbine engine; (5) enabling a PID controller of a control system to request a fuel valve position based upon the defined moderate EGT and the actual EGT gas turbine engine; (6) selecting the requested fuel valve position based upon the request that results in the least amount of fuel entering into the combustion chamber; (7) exiting the control loop if synchronous speed is reached; (8) exiting the control loop if the engine does not reach synchronous speed within a predetermined time in order to shut down and purge the combustion chamber; and (9) repeating steps 1 through 8 until synchronous speed is reached.